docs: def Chisholm_voidage(x, rhol, rhog):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_, as given in [2]_ and [3]_.
  
      .. math::
          \alpha = \left[1 + \left(\frac{1-x}{x}\right)\left(\frac{\rho_g}
          {\rho_l}\right)\sqrt{1 - x\left(1-\frac{\rho_l}{\rho_g}\right)}
          \right]^{-1}
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      [1]_ has not been reviewed. However, both [2]_ and [3]_ present it the
      same way.
  
      Examples
      --------
      >>> Chisholm_voidage(.4, 800, 2.5)
      0.949525900374774
  
      References
      ----------
      .. [1] Chisholm, D. "Pressure Gradients due to Friction during the Flow of 
         Evaporating Two-Phase Mixtures in Smooth Tubes and Channels." 
         International Journal of Heat and Mass Transfer 16, no. 2 (February 1, 
         1973): 347-58. doi:10.1016/0017-9310(73)90063-X.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      '''
      S = (1 - x*(1-rhol/rhog))**0.5
      alpha = (1 + (1-x)/x*rhog/rhol*S)**-1
      return alpha